An organic electroluminescence device (organic EL device) is a promising solid-state emitting type inexpensive and large full-color display device, and has been extensively developed. In general, an organic EL device includes an emitting layer and a pair of opposing electrodes holding the emitting layer therebetween. When an electric field is applied between the electrodes, electrons are injected from a cathode and holes are injected from an anode. The electrons recombine with the holes in the emitting layer to produce an excited state, and energy is emitted as light when the excited state returns to the ground state.
In addition, a phosphorescent organic EL device has been proposed wherein an organic phosphorescent material is used for an emitting layer of the organic EL device. In the phosphorescent organic EL device, high luminous efficiency is achieved by utilizing a singlet state and a triplet state of the excited state of the organic phosphorescent material. When an electron and a hole are recombined in the organic EL device, since it is believed that singlet exciton and triplet exciton are generated at a rate of 1:3 due to the difference in spin multiplicity, three to four times greater luminous efficiency is considered to be achieved if a phosphorescent emitting material is used, in comparison with a device using a fluorescent material only.
Early organic EL devices are insufficient in driving voltage, luminous efficiency and durability, and various technical improvements have been made for the problems.
The improvements of luminous efficiency and lifetime of the organic EL device are important subjects which lead to a low power consumption of display and improvement of durability. Therefore, further improvement is required. In addition, a variety of studies have been carried out to improve luminous efficiency and life time of an organic EL device using a phosphorescent emitting material.
To solve the problems, Patent Document 1 discloses a biscarbazole derivative which can be used as a hole-transporting material. This biscarbazole skeleton has a function to improve heat resistance. Patent Document 2 discloses a biscarbazole derivative which can be used as a phosphorescent emitting host material.
Patent Documents 3 and 4 disclose compounds having carbazole, dibenzofuran, and dibenzothiophene skeletons for a phosphorescent emitting host material.
[Patent Document 1] JP-A-H8-3547
[Patent Document 2] JP-A-2008-135498
[Patent Document 3] JP-A-2007-288035
[Patent Document 4] JP-A-2007-311460